1. Field of the Invention
The present invention relates to compositions and methods for bioremediation of chlorinated biphenyls using dechlorinating microorganisms grown in the presence of stimulating halogenated hydrocarbons, and more particularly, polyhalogenated ethenes.
2. Description of the Related Art
Polychlorinated biphenyls (PCBs) are haloaromatic compounds having exceptional chemical stability. Environmental and toxicological problems caused by the use of PCBs have resulted in restriction of their production under the Toxic Substances Control Act of 1976 and a complete ban of their manufacture by the United States Environmental Protection Agency in 1979. Past disposal practices have resulted in substantial PCB contamination of soils and surface water sediments. As a consequence, in the United States, at least 15% of the PCBs manufactured to date remains in the environment as a highly recalcitrant contaminant. Acute toxicological effects of PCB exposure include chloracne (a skin disease), teratotoxicity, endocrine effects, immunotoxicity, carcinogenicity, and hepatotoxicity (liver damage). The mutagenic and carcinogenic character of PCBs and their suspected role in the reproductive failure of wildlife species are issues of great concern. Further, these compounds bioaccumulate and biomagnify in the fatty tissue of animals in the food web, such as fish, which can affect the human population as a result of food consumption. In sum, the toxicological findings on PCBs and their propensity for bioaccumulation raise concern for the well-being of both humans and wildlife.
Historically, harbor regions have been heavily impacted by the accumulation of polychlorinated biphenyls due to their use in and inadvertent release from naval and industrial applications. Due to their hydrophobic character, PCBs strongly associate with organic carbon, clays and silt that settle into the anaerobic regions of marine sediments.
Although lesser chlorinated PCBs ranging from mono- to hexa-chlorinated congeners can be degraded aerobically, extensively chlorinated congeners (e.g., tetrasubstituted) are not transformed under aerobic conditions. Commercial mixtures of PCBs formerly marketed in the United States under the Aroclor trademark typically contained more than 50 of such extensively-chlorinated congeners. The extent of chlorination of the PCBs varies with the specific commercial mixture. For example, Aroclor 1242 is dominated by tri- and tetrachlorobiphenyls, Aroclor 1260 is dominated by penta-, hexa- and heptachlorobiphenyls, and Aroclor 1268 is dominated by hepta-, octa- and nonachlorobiphenyls. Even less-chlorinated Aroclors contain significant levels of congeners with 5 or more chlorine substituents. For this reason, even a consortium of aerobic bacteria (a consortium being a population of bacteria containing different strains with different congener (degradative) & specificity) cannot remove Aroclor PCB compositions from the environment.
Anaerobic dechlorination of PCBs is a critical step in the biodegradation of these anthropogenic compounds in anaerobic sediments. Thus, in order to completely biodegrade PCBs, both anaerobic and aerobic microorganisms are required since the anaerobic microorganisms dechlorinate more extensively chlorinated PCB congeners recalcitrant to aerobic degradation and only aerobic microorganisms are capable of mineralizing lesser-chlorinated congeners. Knowing which anaerobic microorganisms to stimulate or add to a contaminated sediment or soil was a problem recently solved by the present inventors in copending U.S. patent application Ser. No. 09/860,200, the contents hereby incorporated herein by reference for all purposes.
The present inventors, in U.S. patent application Ser. No. 09/860,200, identified for the first time bacterial catalysts of PCB dechlorination. These PCB dechlorinating bacteria have been designated as bacterium double flank-dechlorinating strain (DF-1) and bacterium ortho-dechlorinating stain (O-17). Both of these microorganisms couple their growth to the dechlorination of PCBs.
Additionally, the present inventors have also identified several other putative PCB dechlorinators that are closely related to DF-1 and O-17. The PCB dechlorinating organisms are phylogenetically most similar, based on 16S rDNA sequence, to Dehalococcoides spp. The Dehalococcoides spp. bacterium designated CBDB1 has been shown to couple its growth to the dechlorination of chlorobenzenes (Adrian, L. et al. 2000. Nature. 408:580-583), while Dehalococcoides ethenogenes couples its growth to the dechlorination of tetrachloroethene (Maymo-Gatell, X. et al. 1997. Science. 276:1568-1571), Both chlorobenzenes and tetrachloroethene have been listed by the U.S. EPA as priority pollutants.
Stimulation of the growth and PCB dechlorinating activity of these organisms is required in order to initiate and sustain good biodegradation of PCBs. Due to this limitation investigators have searched for suitable stimulating agents. PCBs (Wu, Q. et al., 1997. Appl. Environ. Microbiol. 63:4818-4825), polybrominated biphenyls (Wu, Q. et al., 1999. Environ. Sci. Technol. 33:595-602; U.S. Pat. No. 5,227,069), halogenated benzoic acids and halogenated salicylic acids (U.S. Pat. No. 5,484,729) have all been shown to stimulate the microbial dechlorination of PCBs when added to sediments. However, these methods have not been proven suitable for all applications. This is in part due to i) inadequate (limited) stimulation, ii) the application of stimulating agent being environmentally undesirable or iii) the stimulant remaining in the sediment after treatment. Thus, there is a need to in the art for compositions and methods that stimulate degradation of PCBs without the negative side-effects presently known to exist.